The principle of electromagnetic force compensation has wide-ranging applications in a diverse range of weighing instruments that are used in commerce, industry, and in laboratories. This principle has the particular advantage that weighing instruments of enormous measuring accuracy can be realized with it. In this regard, an analytical balance based on the principle of electromagnetic force compensation has the capability to determine, for example, a weighing load of 100 grams with a measurement resolution of 0.01 milligrams—i.e., with a precision of one part in ten million.
A balance or weighing cell of the generic category to which the present invention belongs has a stationary base part, and a load receiver that is movably constrained to the base part and serves to receive the weighing load. A permanent magnet system is also present, is preferably mounted on the base part, has an air gap, a coil which is movably suspended in the air gap and conducts the flow of an electric compensation current, and a force-transmitting mechanism connecting the load receiver to the coil.
A provided optoelectronic position sensor, whose sensor signal corresponds to the travel distance by which the interconnected movable parts of the balance are deflected from a zero position when the load is set on the load receiver, typically includes a light source and a light receiver which are in most cases mounted on the base part with a space interval between them, as well as a shutter vane cutting through the space interval and participating in the deflection of the movable part. The signal of the position sensor is sent to a controller which, in response, regulates the compensation current in such a way that, as a result of the electromagnetic force between the coil and the permanent magnet, the shutter vane and movable parts of the balance that are connected to the shutter vane are brought back to the zero position. In other words, the regulation has the effect that the electromagnetic compensation force counterbalances the weighing load. Given that in accordance with the laws of electromagnetism the magnitude of the coil current and the resultant force are proportionate to each other, the weight of a weighing load placed on the load receiver can be determined through a measurement of the coil current.
Within the above outlined field, the present invention is focused on the optoelectronic position sensor, and in particular on the geometry of the position sensor (i.e., the relative dimensions and spatial relationships of the elements within the position sensor arrangement which encompasses the light source, the light receiver, and the shutter vane). The light receiver includes in most cases a photodiode with at least one light-sensitive area or element. A photodiode is a semiconductor element which, when exposed to light, generates a current which, within a certain range, is proportionate to the amount of incident light.
The shutter vane often has a slit-shaped passage opening, but other shapes of an opening for the passage of light are also possible. For example a circular hole or an elongated hole may be employed. The light receiver can be configured as two separate light-sensitive areas of the photodiode which are operating in a differential circuit arrangement. Deflection of the shutter from its zero position will cause a shift of the illumination image on the light receiver so that one of the light-sensitive areas will receive more light while the other light-sensitive area receives less. Accordingly, in a deflected position of the shutter vane, the respective currents generated by the two light-sensitive areas will be different from each other, wherein the current difference measured by the differential circuit arrangement of the two light-sensitive areas represents the electrical output signal of the light receiver, i.e. the position sensor signal. The functional relationship between the deflection and the electrical position sensor signal is also referred to as the characteristic of the position sensor.
The primary requirement that has to be met by the position sensor of an electromagnetic compensation balance is that the zero position (i.e., the specific position of the shutter vane at which the zero crossing of the sensor signal from negative to positive values takes place) needs to be maintained with the highest degree of accuracy and reproducibility. The zero-point sensitivity (i.e., the slope of the characteristic at its zero crossing) should therefore be as steep as practically possible, so that deflections of the order of nanometers generate a clearly measurable sensor signal.
Further, the graph of the sensor signal plotted over the travel range of the shutter vane (i.e., the characteristic of the position sensor) should be closely reproducible from one weighing cell to the next within the same production run and also for any individual weighing cell when the latter is exposed for example to temperature fluctuations, shocks or vibrations.
Lastly, as another desirable trait, the characteristic of the position sensor should, with good approximation, follow a linear profile. In particular, the sensor signal should be proportional to the deflection of the shutter vane. The requirement for linearity, and more specifically for proportionality, of the sensor signal is among other factors related to the control circuit of the electromagnetic force compensation which is preferably designed as a so-called PID controller—meaning that the compensating force and thus the coil current which is generated as output of the control circuit represents a weighted sum of a component P that is proportionate to the magnitude of the deflection, a component I that is proportionate to the time integral of the deflection, and a component D that is proportionate to the time derivative of the deflection. In order to ensure the respective proportionalities for the three components P, I, D of the coil current, the sensor signal should obviously be as much as possible proportionate to the deflection.
A way of looking at a position sensor as an optical projection system is found for example in patent CH 463 137, wherein an electromagnetic compensation balance is shown with a balance beam carrying at one end a suspended weighing pan, and at the other end a shutter vane with a slit-shaped passage opening which extends into the space interval between a light source and a light receiver. An optical system, represented in rudimentary fashion in FIG. 1 of the aforementioned reference as one lens each arranged in the light path before and after the shutter vane, serves to improve or enhance the optical image of the light source that is projected onto the light receiver. However, this kind of an arrangement of optical lenses in the light path of the position sensor requires appropriately dimensioned (i.e., generally longer) distances from the shutter vane to the light source and to the light receiver—a requirement which would be impossible to meet in particular in weighing cells of a compact, monolithic design. In addition, the production cost would be increased.
A solution presented in U.S. Pat. No. 5,338,902 aims to increase the sensitivity of the position sensor of an electromagnetic compensation balance through mechanical means. The light source and the light receiver are in this case not mounted in a fixed position on the chassis base of the balance, which would be the conventional arrangement, but are instead arranged on a long cantilever arm that is solidly connected to the movable load receiver of the balance, so that the light source and light receiver move up and down together with the load receiver. A two-armed lever which is pivotally supported on the stationary chassis base of the balance is coupled on one side to the load receiver of the balance and carries at the other end the shutter vane, so that the shutter vane moves upward when the load receiver moves downward and vice versa. Thus, the shutter vane moves up and down in unison with the light source and light receiver moving in the opposite direction and as a result there is a larger relative movement of the shutter vane against the light source and light receiver. Consequently, in comparison to the conventional stationary arrangement of the light source and the light receiver, the same amount of deflection of the load receiver causes a stronger position sensor signal to be generated. With this concept, there are again some practical concerns, as the cantilever arm would have to reach through a part of the weighing cell that is in many cases already occupied by the aforementioned force-transmitting mechanism which connects the load receiver to the compensation coil.
In an optoelectronic position sensor according to U.S. Pat. No. 3,805,907, the light source consists of a light-emitting diode, and the light receiver is formed by two phototransistors in a differential circuit arrangement. The phototransistors are arranged diametrically symmetric to each other on the face of a carrier disk that is rotatably mounted on the stationary chassis frame of the balance. By turning the carrier disk, the sensitivity characteristic, i.e. the functional dependency of the sensor signal on the amount of deflection of the shutter vane, can be adjusted. In regard to the geometry of the optical projection system, it is specifically stated that the distance between the two light-sensitive surface portions of the light receiver corresponds to the width of the slit aperture, that the light-emitting surface of the light source lies as close to the shutter vane as possible, that the light-sensitive surface areas of the light receiver are of circular shape, and that the light emitting surface of the light source is somewhat wider than the slit aperture (in particular, 1.5 times the width of the slit aperture). Here, the objection should be raised that the aforementioned characteristic or sensitivity graph of such an arrangement with circular light-sensitive surface portions of the light receiver is in no way linear over the entire deflection range of the shutter vane, but that its slope angle can become, for example, progressively steeper or progressively shallower with an increasing amount of deflection.
The known state of the art includes weighing cells with electromagnetic force compensation which have position sensors where the shutter vane is arranged in a shutter plane approximately midway between the light source and the light receiver. Slight variations in the position of the shutter plane which occur within a production run of weighing cells will lead to a random variation of the sensitivity of the sensor units. It can therefore become necessary with certain products to adjust each unit individually in the production process of the weighing cells, which adds to the manufacturing cost.
The present invention therefore provides a position sensor for a balance that is based on the principle of electromagnetic force compensation, wherein the aforementioned main requirements regarding the accuracy and reproducibility of the zero position as well as the reproducibility and linearity of the sensitivity characteristic are met to a greater extent than with the present state of the art. Ideally, such a position sensor will also be optimally adapted to manufacturing requirements.